The present invention relates to communication networks. More particularly, the present invention relates to a method and apparatus for transporting TDM voice traffic over an ATM network.
Voice traffic has traditionally been transported using a Synchronous Transfer Mode to (STM) network. A network using Time Division Multiplexing (TDM) to combine 64 kilobit per second (kbit/s) Digital Signal 0 (DS0) channels is one example of an STM network. Using TDM, each channel of voice traffic is assigned a specific time period, or TDM channel, configured to let the channel carry a desired maximum amount of voice information. In this way, the STM network provides a high Quality of Service (QOS) because each TDM channel, by design, can handle the maximum amount of voice information. As a result, voice information is generally not lost or delayed. However, since silence periods are still transmitted bandwidth is not used efficiently.
It is also known that voice traffic can be transported via an Asynchronous Transfer Mode (ATM) network. An ATM network uses dedicated-connection switching technology that organizes digital data into 53-byte cells and transmits them over a medium using digital signal technology. Individually, a cell is processed asynchronously relative to other related cells and may be queued before being multiplexed with other cells, from other channels, over a single line, or xe2x80x9clink.xe2x80x9d Because ATM networks are more easily implemented by hardware (rather than software), faster processing speeds are possible. In addition, ATM networks allow for more efficient bandwidth use because different services, such as voice and data, can be statistically multiplexed over the same link.
Generally, 24 DS0 voice traffic channels are combined using TDM into a single domestic T1 circuit, or 30 DS0 channels are combined into a single international E1 circuit. A number of these T1 circuits (or E1 circuits) are then combined using an ATM Adaptation Layer (AAL) to create a virtual circuit. An AAL adaptation layer merely packages higher layer information, such as the T1 or E1 circuit information, into the contents of the 53-byte ATM cell. A number of these virtual circuits are then combined for transport over an ATM network link, such as over a single ATM network xe2x80x9cpipe.xe2x80x9d
To maintain the high quality traditionally associated with STM networks, the AAL1 adaptation layer is used together with Constant Bit Rate (CBR) service (together known as xe2x80x9ccircuit emulationxe2x80x9d). As with TDM, the CBR circuit emulation approach provides a constant guaranteed rate of transfer. That is, a CBR connection allocates enough bandwidth to each channel to support a desired maximum rate of information. In this way, CBR circuit emulation provides a QOS similar to that of an STM network, but does not provide any statistical multiplexing benefits since cells are still used to transport silent periods during a conversation.
To better utilize the bandwidth of an ATM network link, a xe2x80x9ctype 2xe2x80x9d AAL (AAL2) adaptation layer can remove silence periods in voice traffic, known as xe2x80x9csilence suppression,xe2x80x9d and also allows for voice compression. The AAL2 adaption layer can be used, for example, together with a Variable Bit Rate (VBR) virtual circuit. A VBR connection is one over which the rate of transmission can vary. Information is generally sent at a nominal rate of transmission, or Sustainable Cell Rate (SCR), but the rate can exceed this standard by xe2x80x9cburstingxe2x80x9d up to a peak rate, or Peak Cell Rate (PCR). The peak rate can only, however, be momentarily exceeded, as defined by the Maximum Burst Size (MBS). A real-time VBR (rt-VBR) connection is typically used when each end of the connection is required to maintain a timing relationship.
Using AAL2 together with a rt-VBR connection, it is possible to achieve significant statistical multiplexing gains when combining multiple voice channels onto a single virtual circuit. For each rt-VBR virtual circuit, SCR, PCR and MBS values are selected to achieve an acceptable delay objective. If the combined sources violate these parameters, cells may be droppedxe2x80x94resulting in reduced voice traffic quality. Also, cell queuing may take place, which also reduces the quality of voice traffic. In other words, the bandwidth of the rt-VBR virtual circuit is chosen to be less than the maximum bandwidth needed (i.e., the bandwidth needed if all voice channels delivered voice traffic with no silence periods). Therefore, the actual total load can sometimes exceed the virtual circuit bandwidth. This may lead to the loss of ATM cells or an increase in the delay experienced by ATM cells, either of which can reduce the quality of voice traffic.
Moreover, the network must monitor, or xe2x80x9cpolice,xe2x80x9d each rt-VBR virtual circuit to ensure that the traffic does not exceed the allocated SCR, PCR and MBS values. In addition, when a rt-VBR virtual circuit exceeds these values, complex overload control and/or call admission control techniques may be needed to correct the situation and prevent the virtual circuit from adversely impacting the rest of the voice traffic carried over the ATM pipe.
In view of the foregoing, it can be appreciated that a substantial need exists for a method and apparatus to transport TDM voice traffic over an ATM network that allows for statistical multiplexing benefits while providing high quality voice service and solving the other problems discussed above.
The disadvantages of the art are alleviated to a great extent by a method and apparatus for transporting TDM voice traffic over an ATM network. A first plurality of TDM voice traffic circuits, such as T1 or E1 circuits, are multiplexed to create a first rt-VBR virtual circuit such that the bandwidth of the first rt-VBR virtual circuit is not limited. A second plurality of TDM voice traffic circuits are multiplexed to create a second rt-VBR virtual circuit such that the bandwidth of the second rt-VBR virtual circuit is not limited. This may be done by AAL2 multiplexing with substantially large SCR, PCR and MBS values. The first and second rt-VBR virtual circuits are combined for transport over a link in the ATM network. An overload and/or admission control process may be performed based on the ATM network link utilization. The TDM voice traffic AAL2 multiplexing uses silence suppression and may or may not use voice compression.
With these and other advantages and features of the invention that will become hereinafter apparent, the nature of the invention may be more clearly understood by reference to the following detailed description of the invention, the appended claims and to the several drawings attached herein.